The management of trace quantities of chemical species for the purpose of detection frequently involves the extraction of the targeted trace species as a vapor from a carrier medium in which it was originally dispersed. At very low trace density, for example, at concentrations below parts per million where the mode of detection responds to the targeted trace species and also to the accompanying carrier and other species that may be present, the competition of these accompanying materials for the detection events will frequently obscure the desired detection of the targeted trace species.
The terms "targeted trace vapors", "targeted material", and "targeted trace species" will be used interchangeably hereinafter to refer to a particular trace species of interest. Also, the terms "sampled medium" and "sampled mixture" will be used interchangeably hereinafter to refer to the mixture of the carrier medium and all trace species therein.
A common method used to avoid, in part, the vitiation of desired detection of the targeted material caused by the accompanying species in a mixture, is to pass a portion of the sampled mixture containing the targeted material through a separator inlet. A separator inlet is comprised of an aperture or surface which preferentially passes the targeted material at a higher rate while slowing, blocking, or diverting at least some of the other components of the sampled mixture, including in particular the predominant carrier.
A first representative example of such a separator inlet is an effusion separator (FIG. 1a), which includes a microporous cylinder or tube that may be made of sintered glass by way of example, and which is preferentially permeable to small molecules. This would be an appropriate separator if the sampled carrier medium consists exclusively of gases or vapors with the dominant constituent being small molecules of carrier gas. The larger molecules in the sampled medium, including the targeted material, are retained and pass through the entire length of the tube. Most of the smaller molecules are lost from the sample stream by effusion through the permeable walls of the tube and they are subsequently pumped away.
A second representative example of a separator inlet is a jet separator (FIG. 2). It relies on free expansion of a high velocity stream of the sampled medium injected through an inlet jet into a vacuum space. An exit skimmer is mounted coaxial with respect to the inlet jet and positioned close to it. The sampled medium passes through the inlet jet at near supersonic speed. Redistribution of momenta leaves the lighter molecules with more transverse velocity, and they are thereby less likely to enter the exit skimmer than the heavier molecules. The lighter molecules are thereby separated from the exiting sample gas stream and are pumped away.
A third representative example of a separator inlet is a membrane separator. FIG. 1b shows a single membrane separator and FIG. 3a shows a double membrane separator comprising two membranes in series. The membranes are largely impermeable to certain gasses, usually called permanent gasses, such as for example oxygen, nitrogen, and argon. A broad class of substances, hereinafter termed "the sample", that includes the targeted material, permeates the membranes more easily. For selected membranes, this class comprises substances having low polarity and high boiling point characteristic of organic materials. When molecules of the lower polarity, higher boiling point materials contact the inlet side of the membranes, they have a relatively high probability of sticking and being taken into solution in the membranes. Once in solution, they diffuse through the membranes at a rate proportional to the gradient of their concentration. They emerge at the outlet side of the membranes substantially separated from the carrier gas constituents. Most of the carrier that permeates the first membrane 26 is pumped away at the interstage, while more than one-half of the sample progresses to the second membrane 28. The output of the second membrane 28 can exit the overall separator 24 to a detector inlet at sub atmospheric pressure as shown in FIG. 3a or to an interior gas circuit 37 as shown in FIG. 3b. In either case, it is necessary to quickly remove the sample molecules from contact with the output side of the membrane in order to preserve the partial pressure gradient that drives their transport through the membrane.
Although utilizing a separator inlet enables an increase in the concentration ratio of the targeted material and is a form of pre-concentration in the sense that the ratio of the mass of the targeted material to the mass of the very small amount of carrier medium transmitted is increased, its utilization does not increase the density or partial pressure of the targeted material on a continuous basis at the separator output.
In earlier detection systems, pre-concentrating the sample, as heretofore described, generally improved sensitivity of the detection by reducing the ratio of permanent gasses to sample species introduced into the detector. However, the extent of improvement has been strongly dependent on the type of detector and the detailed manner in which the sample has been introduced and manipulated therein.
For successful detection in any practical system, not only must the quantity of the carrier gas be reduced, but also the quantity of targeted material in the sample must be above some minimum amount. In addition, the condition of the sample must be matched to the inlet requirement of the detection system within some tolerable range. The most relevant measure of the required condition of the sample is the density or partial pressure of the targeted material when introduced to the detection system. This measure can usually be translated to a specification of minimum detectability.
In a co-pending patent application incorporated herein below by reference, I describe another of my developments involving a near-real-time detection system using a gas chromatograph/mass spectrometer (GC/MS) tandem instrument to detect targeted materials dispersed in ambient air. To satisfy the input requirements of this advantageous detection method and to provide the desired detection at very low target ambient concentrations, substantial pre-concentration is needed first of all to match the impulse injection requirements of the GC and further to provide a sufficient mass flow rate of the targeted species for the MS to detect. In samples collected from the ambient, at very low trace detection levels for the targeted species, there will generally be higher concentrations of other trace component species, whose presence may obscure the desired detection in the MS. The function of the GC in the tandem GC/MS instrument is to provide a serial elution of the various species in the sample injected into the column after the separator has extracted the family of many trace constituents in the sampled medium from the carrier medium. The MS can then examine the targeted species in a unique time window with minimum interference from interfering species. The GC requires a very short pulse injection of the acquired sample in a minimum quantity of carrier gas, thus imposing a severe burden of pre-concentration on the management of the sample.
The older more traditional GC/MS technique, illustrated in FIG. 4, has been the analytical chemist's method of choice for analyzing certain types of chemical mixtures in the laboratory. In a traditional GC/MS system, the GC column separates sample components in their respective time of elution, and the components are individually sequentially analyzed by the mass spectrometer to provide definitive identification. Despite the exceptional capability of the traditional GC/MS system for analyzing trace levels in mixtures in the laboratory, its application to near-real-time detection of targeted materials dispersed in an ambient carrier has severe limitations. First, traditional GC has been generally characterized by elution times of many minutes and even hours to elute the components of mixtures into separate time windows, thereby defeating near-real-time performance. Second, to carry out the needed separation of the components of the mixture, the entire sample to be analyzed must be introduced into the GC in an impulse time that is less than the differences in elution times of distinguishable components. Third, the GC inlet is traditionally a closed port that excludes air. The sample is typically transported from the output side of the membrane to the GC by a carrier gas that contains no oxygen because, for many GC columns, the stationary phase can be degraded by oxygen, particularly at higher temperatures.
The limitations of the traditional GC/MS mentioned above, especially the low concentration of the targeted material in the ambient, work directly against the requirements of a system for making the near-real-time trace vapor detection. More recent GC/MS implementation employs capillary small-bore columns that are capable of short elution separation times. However, this exacerbates the requirement for the very short injection time required for GC operation. For ultra low concentration ratios, if just a very small increment of sample bearing carrier is injected in accord with the requirement for short impulse injection, the total amount of the targeted material will be below the minimum discernable limits of sensitivity of the MS detector. By way of example, gas or vapor injections into a nominal capillary GC column, scaled for the fast elution times required, must be less than about 10.sup.-5 liter in volume. Trace vapor samples, for example of molecular weight three-hundred, sought to be detected at 0.1 part per billion, will be present in the ambient at levels of about 1.36.times.10.sup.-9 grams per liter. This would allow an injection of only 1.36.times.10.sup.-14 grams, which is below the practical detection limit of the MS.
This example clearly illustrates the severe mismatch of the trace vapor sample condition and the inlet requirement of a near-real-time GC/MS trace vapor detector. My recent work, incorporated herein by reference, which is the subject of a co-pending patent application, Ser. No. 08/738,961, entitled "Real Time Trace Vapor Detection" of which I am the sole inventor, and which is assigned to the same assignee as the present invention, relates to a capillary GC/MS system which has been modified to adapt it for near-real-time detection of trace vapors in ambient air.
The aforementioned modifications disclosed in said co-pending application are comprised of; 1) providing an inlet membrane separator to concentrate a sample mixture of trace substances from the ambient air and to deliver the concentrated sample mixture to a suitable interior carrier gas stream; 2) providing a miniature vapor collection cold trap operating as a "micro accumulator" to capture the mixture of trace substances; and 3) providing a short thermal pulse to the cold trap, resulting in the impulse injection of the trace substances into the GC/MS system, wherein a fast GC/MS analysis is enabled.
Whereas the modified system heretofore described makes real-time GC/MS trace vapor detection and analysis possible, the system sensitivity is not sufficient for fast near-real-time response when dealing with ultra low concentration trace vapors dispersed in air. Greater density of the trace vapor samples in the carrier medium in short periods of time is required to enable greater sensitivity.